Angle-resolved antisymmetric scatterometry

ABSTRACT

A method for determining an overlay offset may include, but is not limited to: obtaining a first anti-symmetric differential signal (ΔS 1 ) associated with a first scatterometry cell; obtaining a second anti-symmetric differential signal (ΔS 2 ) associated with a second scatterometry cell and computing an overlay offset from the first anti-symmetric differential (ΔS 1 ) signal associated with the first scatterometry cell and the second anti-symmetric differential signal (ΔS 2 ) associated with the second scatterometry cell.

RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.13/386,524 filed Jan. 23, 2012 which claims priority to PatentCooperation Treaty Application No. PCT/US10/42738 filed on Jul. 21, 2010which claims priority to U.S. Provisional Application Ser. No.61/227,722 filed on Jul. 22, 2009, all of which are hereby incorporatedby reference in their entirety, to the extent not inconsistent herewith.Further, U.S. Pat. No. 7,277,172 is incorporated by reference in itsentirety to the extent not inconsistent herewith.

BACKGROUND

Scatterometry is currently used in the semiconductor industry to measurethickness and optical properties of thin films as well as the criticaldimension (CD) and profile shape of periodic structures on asemiconductor wafer. In principle, scatterometry has clear advantagesover the current imaging technology of overlay metrology. Scatterometrymay be capable of measuring device-size structures that cannot beresolved by imaging. Scatterometry may be also thought to be more robustto process variations and asymmetry in the profile of the measuredstructure.

Methods for measuring profile asymmetry include critical dimensionscanning electron microscopy (CD-SEM) and scatterometry. The CD-SEMapproach may be very slow and expensive. The current implementation ofscatterometry CD metrology, which may be also suitable for monitoringproperties of the profile (including profile asymmetry), relies ondetailed modeling and may be therefore also rather slow. In addition, itmay be very difficult to accurately model complicated profiles, such astwo gratings (one on top of the other) separated by a layered possiblynon-flat film.

SUMMARY

A method for determining an overlay offset may include, but is notlimited to: obtaining a first anti-symmetric differential signal (ΔS₁)associated with a first scatterometry cell; obtaining a secondanti-symmetric differential signal (ΔS₂) associated with a secondscatterometry cell and computing an overlay offset from the firstanti-symmetric differential (ΔS₁) signal associated with the firstscatterometry cell and the second anti-symmetric differential signal(ΔS₂) associated with the second scatterometry cell.

It may be to be understood that both the foregoing general descriptionand the following detailed description may be exemplary and explanatoryonly and may be not necessarily restrictive of the invention as claimed.The accompanying drawings, which may be incorporated in and constitute apart of the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich Figure Number:

1 shows a patterned scatterometry cell;

2A shows a scatterometry target including patterned scatterometry cells;

2B shows a scatterometry target including patterned scatterometry cells;

2C shows a scatterometry target including patterned scatterometry cells;

3 shows a scatterometry system;

4 shows a scatterometry system;

5A shows polarizer configurations;

5B shows polarizer configurations;

6A shows polarizer configurations;

6B shows polarizer configurations;

7A shows an illumination pupil;

7B shows a collection pupil;

8A shows an illumination pupil;

8B shows a collection pupil;

9A shows an illumination pupil;

9B shows a collection pupil;

10 shows a method for conducting scatterometry;

11 shows a method for conducting scatterometry;

12 shows a method for conducting scatterometry;

13 shows a method for conducting scatterometry;

14 shows a method for conducting scatterometry;

15 shows a method for conducting scatterometry;

16 shows a method for conducting scatterometry;

17 shows a method for conducting scatterometry;

18 shows a method for conducting scatterometry;

19A shows a scatterometry system; and

19B shows a scatterometry system.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of exemplary embodiments,reference may be made to the accompanying drawings, which form a parthereof. In the several figures, like referenced numerals identify likeelements. The detailed description and the drawings illustrate exemplaryembodiments. Other embodiments may be utilized, and other changes may bemade, without departing from the spirit or scope of the subject matterpresented here. The following detailed description may be therefore notto be taken in a limiting sense, and the scope of the claimed subjectmatter may be defined by the appended claims. It will be understood thatthe following description may be not intended to limit the invention tothe described embodiments. To the contrary, it may be intended to coveralternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.

This invention is concerned with the measurement of differential-signalscatterometry overlay (SCOL). An overlay target may comprise one or morecells each having at least two patterns between which overlay ismeasured. Each cell may comprise two largely overlapping patterns, eachof which may be in the same or different layers of the wafer. Thegeometric designs of the cells (pattern pitch, CD, etc.) may beidentical, except for a programmed offset between the two patterns,which may vary from cell to cell. The two patterns may be referred to asthe top and bottom patterns, and the overlay information refers to theshift between the top and bottom patterns. An exemplary overlay targetis illustrated in FIG. 1. A top pattern 120 and a bottom pattern 121 maybe shifted relative to each other by a total offset (v) including theprogrammed offset and any fabrication related overlay.

The total offset v of a cell may be the sum of two contributing factors:a programmed offset and an overlay offset (if present). The programmedoffset may be designed into the cell. The overlay offset may be anunknown quantity to be determined which results from fabricationvariations. Hence, in preparation for a scatterometry measurementaccording to an embodiment of the present invention, a target may bedesigned to comprise a plurality of cells, each of which has aprogrammed offset contributing to its total offset. Particular cells mayhave grating orientations orthogonal to one another (e.g. a first“horizontal” cell and a second “vertical” cell), for independentmeasurement of overlay in an x-direction and overlay in a Y-direction.

A scatterometry signal may be measured for each cell, and thendifferences between signals are calculated from which the value of theoverlay may be extracted. The advantage of this method over otherscatterometry methodologies is that the measurement of the overlayrelies on symmetry characteristics between the measured signalsthemselves rather than a detailed comparison of the measured signalswith simulated reference signals. As a result, no regression on orgeneration of a large library of simulated reference signals isrequired. However, such differential methodologies may require a largetarget consisting of several cells. As an example, a SCOL target mayconsist of 8 cells, 4 for X-overlay measurement and 4 for Y-overlay.

The architectures of this invention provide for multiple measurements oneach cell in specific configurations that enable the extraction of oneor more differential signals from each cell instead of a differentialsignal from a pair of cells. This may serve to reduce the number ofcells required per target and hence the size of the target.

For example, two inspection signals may be employed to examine anoverlay target. Inspection signal responses may be obtained for eachinspection signal for each cell. It may be the case that a differentialbetween a signal response associated with a first inspection signal andsignal response associated with a second inspection signal may exhibitanti-symmetric properties as follows:ΔS(offset)=−ΔS(−offset)

Such differentials may vanish at offsets of zero and may be linearfunctions of offset for small values of the offset. The slope of suchlinear functions along with the overlay offset in a particular directioncan be determined from measurements on two cells having differentprogrammed offsets (i.e. two measured differential signals may be usedto determine unknown slopes and overlays). As such, it may be desirableto provide systems and methods to generate signal responses exhibitingsuch anti-symmetric behavior to facilitate offset computation.

For example, as shown in FIG. 2A, an overlay target 113 may includemultiple cells 127. The cells 127 may be arranged as cell pairs 128. Thegratings of the cells 127 of cell pair 128A may be orthogonal to thegratings of the cells 127 of cell pair 128B to allow overlay measurementin both the x-direction and the y-direction. Each cell 127A may have aprogrammed offset v₁ between its top and bottom gratings and each cell127B may have a programmed offset v₂, where v₁≠v₂. The combined use ofan illumination beam having a first polarization and an illuminationbeam having a second polarization (as described below) to illuminate anoverlay target 113 having such cell configurations may result in signalresponse differentials for each cell 127 which exhibit anti-symmetricbehavior. Such anti-symmetric behavior may allow for zero-orderscatterometry to be performed with only two cells 127 per direction asadditional orthogonal information is provided by the sequentialillumination of the overlay target 113 with the alternate polarizationstates.

In a particular configuration shown in FIG. 2B, a first scatterometrycell 127A may include two-dimensional grating pattern having a firstprogrammed offset (e.g. v₁) in a first direction (e.g. in thex-direction) and a second non-zero programmed offset (e.g. v₂) in asecond direction orthogonal to the first direction (e.g. in ay-direction). Further, a second scatterometry cell 127B may include atwo-dimensional grating pattern having a first programmed offset (e.g.v₃) in the first direction (e.g. in the x-direction) unequal to thefirst programmed offset of the first scatterometry cell (e.g. v₁≠v₃) anda second non-zero programmed offset (e.g. v₄) in the second direction(e.g. in the y-direction) equal to the second programmed offset of thefirst scatterometry cell (e.g. v₂=v₄).

In an alternate configuration shown in FIG. 2C, an overlay target 113may include a first cell 127A, a second cell 127B and a third cell 127C.Each cell 127 may include a two-dimensional grating. The three cells 127may each have programmed offsets in both the x-direction and they-direction. The programmed offsets may be configured such that themeasurement of overlay offset in the x-direction utilizes cell 127A andcell 127B while the measurement of overlay offset in the y-directionutilizes cell 127B and cell 127C. The programmed offset of a cell i inthe x-direction and the y-direction may be noted as v_(i) ^((x)), andv_(i) ^((y)), respectively. A particular example of the respectiveprogrammed offsets of a three-cell configuration may be as follows:v ₁ ^((x)) =v ₂ ^((x)) =−v ₃ ^((x)) =f ₀ ^((x)) ;v ₁ ^((y)) =v ₂ ^((y))=−v ₃ ^((y)) =f ₀ ^((y))where f₀ ^((x)) and f₀ ^((y)) are the fundamental offsets in thex-direction and the y-direction, respectively. The fundamental offsetsf₀ ^((x)) and f₀ ^((y)) may be significantly smaller than the patternpitches for the gratings of the scatterometry cells 127. For example,the fundamental offsets f₀ ^((x)) and f₀ ^((y)) may be of the order offrom 10 nm-15 nm.

FIG. 3 shows a high-level block diagram illustrating a scatterometrysystem. The system may comprise a polarizer 111 on the illumination sideand a polarizer 115 on the collection side. The polarizer 111 and thepolarizer 115 may be configured to impart various polarizations onincident light. The incident light 118 emitted from a light source 110may travel through the polarizer 111 and focusing optics 112, and may bescattered by a sample. The sample may be a semiconductor wafercomprising at least one overlay target 113. Interaction of the incidentlight 118 with one or more scatterometry cells 127 of an overlay target113 may scatter incident light 118 thereby modifying the state ofpolarization of the incident light 118. The scattered light 119 maytravel through collection optics 114 and a polarizer 115 where it may bedetected by detector 116.

FIG. 4 shows another diagrammatic representation of a configuration of ascatterometry system showing the change in the state of lightpolarization before and after interaction with a scatterometry cell 127of an overlay target 113, and showing a charge-coupled device (CCD)detector 116 for detecting the scattered light 119. For example, thepolarizer 111 and may be configured to impart a first polarization tothe incident light 118A resulting in polarized light 118B (e.g.vertically polarized light). The polarizer 115 may impart a secondpolarization to the scattered light 119A. Various polarizationcombinations for the polarizer 111 and the polarizer 115 may beemployed. For example the polarizer 111 and the polarizer 115 may impartthe following respective polarizations: horizontal/vertical,vertical/horizontal, azimuthal/radial, and/or radial azimuthal.

Anti-symmetric differential signals used to compute the target overlayoffset may be obtained by conducting two measurements on a cellutilizing various optical configurations of an illumination path and acollection path of a scatterometry system as described below. Referringto FIG. 10, a method 1000 for estimating overlay offset of a sampleusing anti-symmetric differential signals is illustrated.

The operational flows, discussion and explanation may be provided withrespect to the exemplary FIGS. 1-9B, and/or with respect to otherexamples and contexts. It should be understood that the operationalflows may be executed in a number of other environments and contexts,and/or in modified versions of FIGS. 1-9B. In addition, although thevarious operational flows are presented in the sequence(s) illustrated,it should be understood that the various operations may be performed inorders and combinations other than those that are illustrated. Forexample, any operational step may be combined with any other operationalstep in any order.

Referring to FIG. 10, operation 1010 depicts obtaining a firstanti-symmetric differential signal (ΔS₁) associated with a firstscatterometry cell. For example, as shown in FIGS. 1-4, a light source110 may illuminate a first scatterometry cell 127A of a cell pair 128with a first beam of incident light 118 (e.g. a beam having a firstpolarization) to generate scattered light 119. A CCD detector 116 mayreceive the scattered light 119 and convert that light into electricalsignals to be processed by a computing device 126. Further, the lightsource 110 may illuminate the first scatterometry cell 127A of a cellpair 128 with a second beam of incident light 118 (e.g. a beam having asecond polarization) to generate scattered light 119. A CCD detector 116may receive the scattered light 119 and convert that light intoelectrical signals to be processed by a computing device 126. Thecomputing device 126 may receive the electronic signal responsesassociated with each illumination of the scatterometry cell 127A andcompute a differential between the signals.

Operation 1020 depicts obtaining a second anti-symmetric differentialsignal (ΔS₂) associated with a second scatterometry cell. For example,as shown in FIGS. 1-4, a light source 110 may illuminate a secondscatterometry cell 127B of a cell pair 128 with a first beam of incidentlight 118 (e.g. a beam having a first polarization) to generatescattered light 119. A CCD detector 116 may receive the scattered light119 and convert that light into electrical signals to be processed by acomputing device 126. Further, the light source 110 may illuminate thesecond scatterometry cell 127B of a cell pair 128 with a second beam ofincident light 118 (e.g. a beam having a second polarization) togenerate scattered light 119. A CCD detector 116 may receive thescattered light 119 and convert that light into electrical signals to beprocessed by a computing device 126. The computing device 126 mayreceive the electronic signal responses associated with eachillumination of the scatterometry cell 127B and compute a differentialbetween the signals.

Operation 1030 depicts computing an overlay offset from the firstanti-symmetric differential (ΔS₁) signal associated with the firstscatterometry cell and the second anti-symmetric differential signal(ΔS₂) associated with the second scatterometry cell. For example, asshown at Operation 1110, in the case of a first cell 127A and a secondcell 127B of a cell pair 128A having programmed offsets v₁=f₀ and v₁=−f₀in a given direction and differential signals ΔS₁ (as determined inOperation 1010) and ΔS₂ (as determined in Operation 1020) which areanti-symmetric, if both the overlay and the programmed offsets aresmall, it may be assumed that the differential signals are linearfunctions of the total offsets:ΔS ₁ ∝f ₀+overlay;ΔS ₂ ∝−f ₀+overlay.Therefore, the overlay offset of a cell pair 128 (e.g. along thex-direction) may be computed as:

${{overlay}\mspace{14mu}{offset}} = {f_{0} \cdot {\frac{{\Delta\; S_{1}} + {\Delta\; S_{2}}}{{\Delta\; S_{1}} - {\Delta\; S_{2}}}.}}$

In one embodiment, obtaining a first anti-symmetric differential signal(ΔS₁) associated with a first scatterometry cell of operation 1010 mayinclude additional operations 1210 and 1220, and obtaining a secondanti-symmetric differential signal (ΔS₂) associated with a secondscatterometry cell of operation 1020 may include additional operations1230 and 1240 as shown in FIG. 12.

Operation 1210 depicts obtaining scattering signals generated by a firstscatterometry cell in response to illumination by a first illuminationbeam and scattering signals generated by the first scatterometry cell inresponse to illumination by a second illumination beam. For example, asshown in FIGS. 1-4, a light source 110 may illuminate a firstscatterometry cell 127A of a cell pair 128 with a first beam of incidentlight 118 (e.g. a beam having a first polarization) to generatescattered light 119. A CCD detector 116 may receive the scattered light119 and convert that light into electrical signals to be processed by acomputing device 126. Further, the light source 110 may illuminate thefirst scatterometry cell 127A of a cell pair 128 with a second beam ofincident light 118 (e.g. a beam having a second polarization) togenerate scattered light 119. A CCD detector 116 may receive thescattered light 119 and convert that light into electrical signals to beprocessed by a computing device 126.

Operation 1220 depicts computing a differential signal (ΔS₁) between:the scattering signals generated by the first scatterometry cell inresponse to illumination by the first illumination beam and thescattering signals generated by the first scatterometry cell in responseto illumination by the second illumination beam. For example, thecomputing device 126 may receive first electronic signals from thedetector 116 generated in response to scattering of a first illuminationbeam (e.g. a beam having a first polarization) by a first cell 127A of acell pair 128. The computing device 126 may further receive secondelectronic signals from the detector 116 generated in response toscattering of a second illumination beam (e.g. a beam having a secondpolarization) by the first cell 127A of a cell pair 128. The computingdevice 126 may carry out one or more optimization computations on thefirst and second electronic signals (e.g. normalization, subtraction ofdark signals, etc.) to remove background effects, noise, and the like.Following such computations, the computing device 126 may then compute adifferential between the first electronic signals and the secondelectronic signals.

Operation 1230 depicts obtaining scattering signals generated by asecond scatterometry cell in response illumination by a firstillumination beam and scattering signals generated by the secondscatterometry cell in response to illumination by a second illuminationbeam. For example, as shown in FIGS. 1-4, a light source 110 mayilluminate a second scatterometry cell 127B of a cell pair 128 with afirst beam of incident light 118 (e.g. a beam having a firstpolarization) to generate scattered light 119. A CCD detector 116 mayreceive the scattered light 119 and convert that light into electricalsignals to be processed by a computing device 126. Further, the lightsource 110 may illuminate the second scatterometry cell 127B of a cellpair 128 with a second beam of incident light 118 (e.g. a beam having asecond polarization) to generate scattered light 119. A CCD detector 116may receive the scattered light 119 and convert that light intoelectrical signals to be processed by a computing device 126.

Operation 1240 depicts computing a second differential signal (ΔS₂)between: the scattering signals generated by the second scatterometrycell in response to illumination by the first illumination beam and thescattering signals generated by the second scatterometry cell inresponse to illumination by the second illumination beam. For example,the computing device 126 may receive first electronic signals from thedetector 116 generated in response to scattering of a first illuminationbeam (e.g. a beam having a first polarization) by a second cell 127B ofa cell pair 128. The computing device 126 may further receive secondelectronic signals from the detector 116 generated in response toscattering of a second illumination beam (e.g. a beam having a secondpolarization) by the second cell 127B of a cell pair 128. The computingdevice 126 may then compute a differential between the first electronicsignals and the second electronic signals.

In an embodiment, obtaining scattering signals generated by a firstscatterometry cell in response to illumination by a first illuminationbeam and scattering signals generated by the first scatterometry cell inresponse to illumination by a second illumination beam of operation 1210may include additional operations 1310-1340, and obtaining scatteringsignals generated by a second scatterometry cell in responseillumination by a first illumination beam and scattering signalsgenerated by the second scatterometry cell in response to illuminationby a second illumination beam of operation 1230 may include additionaloperations 1350-1380 as shown in FIG. 13.

Operation 1310 depicts illuminating the first scatterometry cell with anillumination beam having a first polarization. For example, as shown inFIGS. 2-6B, the polarizer 111 may be configured to impart a firstpolarization to the incident light 118A resulting in polarized light118B having the first polarization. The rotational angle of thepolarizer 111 may be adjusted to specify the particular polarizationconfiguration. The polarized light 118B may be directed towards a firstcell 127A of a cell pair 128 (e.g. cell pair 128A) of an overlay target113 by focusing optics 112 so as to illuminate the first cell 127A. Thepolarized light 118B may be scattered by the first cell 127A resultingin scattered light 119A.

Operation 1320 depicts imparting a second polarization to scatteringsignals generated by the first scatterometry cell in response toillumination by the illumination beam having the first polarization. Forexample, as shown in FIGS. 2-6B, the polarizer 115 may be configured toimpart a second polarization to the scattered light 119A resulting inpolarized scattered light 119B having the second polarization. Therotational angle of the polarizer 111 may be adjusted to specify theparticular polarization configuration.

Operation 1330 depicts illuminating the first scatterometry cell with anillumination beam having the second polarization. For example, as shownin FIGS. 2-6B, the polarizer 111 may be configured to impart the secondpolarization previously imparted to the scattered light 119A by thepolarizer 115 (e.g. as in Operation 1015) to the incident light 118Aresulting in polarized light 118B having the second polarization. Therotational angle of the polarizer 111 may be adjusted to specify theparticular polarization configuration. The polarized light 118B havingthe second polarization may be directed towards the first cell 127A ofthe overlay target 113 by focusing optics 112 so as to illuminate thefirst cell 127A. The polarized light 118B may be scattered by the firstcell 127A resulting in scattered light 119A.

Operation 1340 depicts imparting the first polarization to scatteringsignals generated by the first scatterometry cell in response toillumination by an illumination beam having the second polarization. Forexample, as shown in FIGS. 2-6B, the polarizer 115 may be configured toimpart the first polarization previously applied to the incident light118A by the polarizer 111 (e.g. as in Operation 1010) to the scatteredlight 119A resulting in polarized scattered light 119B having the firstpolarization. The rotational angle of the polarizer 115 may be adjustedto specify the particular polarization configuration.

Referring to FIG. 5A, in a first scatterometry measurement associatedwith a first scatterometry cell (as previously described with respect tooperations 1310-1320), the polarizer 111 may be configured to impart afirst linear polarization (e.g. vertical polarization) to the incidentlight 118A and the polarizer 115 may be configured to impart a secondlinear polarization orthogonal to the first linear polarization (e.g.horizontal polarization) to the scattered light 119A. Referring to FIG.5B, in a second scatterometry measurement associated with the firstscatterometry cell (as previously described with respect to operations1330-1340), the polarizer 111 may be configured to impart the secondlinear polarization (e.g. horizontal polarization) to the incident light118A and the polarizer 115 may be configured to impart the first linearpolarization orthogonal to the second polarization (e.g. verticalpolarization) to the scattered light 119A.

Alternately, referring to FIG. 6A, in a first scatterometry measurementassociated with a first scatterometry cell (as previously described withrespect to operations 1310-1320), the polarizer 111 may be configured toimpart azimuthal polarization to the incident light 118A and thepolarizer 115 may be configured to impart a radial polarization to thescattered light 119A. Referring to FIG. 6B, in a second scatterometrymeasurement associated with the first scatterometry cell (as previouslydescribed with respect to operations 1330-1340), the polarizer 111 maybe configured to impart the radial polarization to the incident light118A and the polarizer 115 may be configured to impart the azimuthalpolarization to the scattered light 119A.

Operation 1350 depicts illuminating the second scatterometry cell withan illumination beam having the first polarization. For example, asshown in FIGS. 2-6B, polarizer 111 may be configured to impart a firstpolarization to the incident light 118A resulting in polarized light118B having the first polarization. The rotational angle of thepolarizer 111 may be adjusted to specify the particular polarizationconfiguration. The polarized light 118B may be directed towards a secondcell 127B of a cell pair 128 (e.g. cell pair 128A) of an overlay target113 by focusing optics 112 so as to illuminate the second cell 127B. Thepolarized light 118B may be scattered by the second cell 127B resultingin scattered light 119A.

Operation 1360 depicts imparting the first polarization to scatteringsignals generated by the second scatterometry cell in response toillumination by an illumination beam having the second polarization. Forexample, as shown in FIGS. 2-6B, polarizer 115 may be configured toimpart a second polarization to the scattered light 119A resulting inpolarized scattered light 119B having the second polarization. Therotational angle of the polarizer 111 may be adjusted to specify theparticular polarization configuration.

Operation 1370 depicts illuminating the second scatterometry cell withan illumination beam having the second polarization. For example, asshown in FIGS. 2-6B, the polarizer 111 may be configured to impart thesecond polarization previously imparted to the scattered light 119A bythe polarizer 115 (e.g. as in Operation 1035) to the incident light 118Aresulting in polarized light 118B having the second polarization. Therotational angle of the polarizer 111 may be adjusted to specify theparticular polarization configuration. The polarized light 118B havingthe second polarization may be directed towards the second cell 127B ofthe overlay target 113 by focusing optics 112 so as to illuminate thesecond cell 127. The polarized light 118B may be scattered by the secondcell 127B resulting in scattered light 119A.

Operation 1370 depicts imparting the first polarization to scatteringsignals generated by the second scatterometry cell in response toillumination by an illumination beam having the second polarization. Forexample, as shown in FIGS. 2-6B, polarizer 115 may be configured toimpart the first polarization previously applied to the incident light118A by the polarizer 111 (e.g. as in Operation 1030) to the scatteredlight 119A resulting in polarized scattered light 119B having the firstpolarization. The rotational angle of the polarizer 115 may be adjustedto specify the particular polarization configuration.

Referring to FIG. 5A, in a first scatterometry measurement associatedwith a second scatterometry cell (as previously described with respectto operations 1350-1360), the polarizer 111 may be configured to imparta first linear polarization (e.g. vertical polarization) to the incidentlight 118A and the polarizer 115 may be configured to impart a secondlinear polarization orthogonal to the first linear polarization (e.g.horizontal polarization) to the scattered light 119A. Referring to FIG.5B, in a second scatterometry measurement associated with the secondscatterometry cell (as previously described with respect to operations1370-1380), the polarizer 111 may be configured to impart the secondlinear polarization (e.g. horizontal polarization) to the incident light118A and the polarizer 115 may be configured to impart the first linearpolarization orthogonal to the second polarization (e.g. verticalpolarization) to the scattered light 119A.

Alternately, referring to FIG. 6A, in a first scatterometry measurementassociated with a first scatterometry cell (as previously described withrespect to operations 1350-1360), the polarizer 111 may be configured toimpart azimuthal polarization to the incident light 118A and thepolarizer 115 may be configured to impart a radial polarization to thescattered light 119A. Referring to FIG. 6B, in a second scatterometrymeasurement associated with the first scatterometry cell (as previouslydescribed with respect to operations 1370-1380), the polarizer 111 maybe configured to impart the radial polarization to the incident light118A and the polarizer 115 may be configured to impart the azimuthalpolarization to the scattered light 119A.

In an embodiment, obtaining scattering signals generated by a firstscatterometry cell in response to illumination by a first illuminationbeam and scattering signals generated by the first scatterometry cell inresponse to illumination by a second illumination beam of operation 1210may include additional operations 1410-1420, and obtaining scatteringsignals generated by a second scatterometry cell in responseillumination by a first illumination beam and scattering signalsgenerated by the second scatterometry cell in response to illuminationby a second illumination beam of operation 1230 may include additionaloperations 1430-1440 as shown in FIG. 14.

In one embodiment, obtaining a first anti-symmetric differential signal(ΔS₁) associated with a first scatterometry cell of operation 1010 mayinclude additional operations 1410 and 1420, and obtaining a secondanti-symmetric differential signal (ΔS₂) associated with a secondscatterometry cell of operation 1020 may include additional operations1430 and 1440 as shown in FIG. 12.

Operations 1410 and 1430 depict routing an illumination beam having atleast one of the first polarization and the second polarization throughat least one waveplate. Operation 1420 depicts routing one or morescattering signals through the at least one waveplate. For example, asshown in FIG. 19A, anti-symmetric differential signals may be obtainedwith an optical configuration that includes a polarizer 111 in theillumination path, a waveplate 130A (e.g. a quarter waveplate) in theillumination path, a waveplate 130B (e.g. a quarter waveplate) in thecollection path and a polarizer 115 in the collection path. Thepolarizers and waveplates may be configured such that differentialsignals measured with proper angles of the polarizers and waveplatesobey the anti-symmetry condition. For example, the angle of thepolarizer 111 in the illumination path may be 0 degrees in a firstmeasurement of a cell and 90 degrees in the second measurement of acell. The angle of the polarizer 115 in the collection path may be 90degrees in the first measurement of the cell and 0 degrees in the secondmeasurement of the cell. The angle of the waveplate 130A in theillumination path may be 45 degrees for both measurements of the cell.The angle of the waveplate 130B in the collection path may be −135degrees for both measurements of the cell. An advantage of such ameasurement scheme is that it allows flexibility in the polarization andphase content of the signal for optimal sensitivity to overlay. Inaddition, such an optical configuration can be tuned to have optimaloverlay sensitivity for relatively small angles of incidence. In thiscase, illumination pupil 122 apodization by an apodizing aperture 131may be used to reduce spot size without loss of sensitivity to overlay.

In an alternate configuration shown in FIG. 19B, the anti-symmetricdifferential signals may be obtained with an optical configuration thatincludes a polarizer 111 in the illumination path, a polarizer 115 inthe collection path and at least two waveplates 130 (e.g. a halfwaveplate 130A a quarter waveplate 130B) in a location common to theillumination path and the collection path, so that both waveplates 130are passed through twice (e.g. once by the incident light 118 beforeilluminating the wafer and a second time by the scattered light 119following scattering of the incident light 118 by a scatterometry cell127). For example, for a first measurement of a cell the polarizer 111may be at 0 degrees, the half waveplate 130A at 205 degrees, the quarterwaveplate 130B at 5 degrees and the polarizer 115 at 90 degrees. For asecond measurement of the cell, the polarizer 111 may be at 90 degrees,the half waveplate 130A at 300 degrees, the quarter waveplate 130B at 15degrees and the polarizer 115 at 0 degrees.

In an alternate embodiment, method 1000 may include an additionaloperation 1510 as shown in FIG. 15.

Operation 1510 depicts providing one or more illumination beams from asource. For example, the light source 110 may emit an incident light118. As shown in FIG. 7A, the light source 110 may include anillumination pupil 122 having an annular portion 123. Use of anillumination pupil 122 including the annular portion 123 may allow forscattered signals to be measured at multiple positions in the pupil viaspecular reflection (i.e. zero-order diffraction). Alternately, as shownin FIG. 8A, a full illumination pupil 122 may be employed. Alternately,as shown in FIG. 9A, an apodized illumination pupil 122 may be employed.Further, incident light 118 may be a monochromatic illumination beam oran illumination beam including a sequence of multiple wavelengths.

In an alternate embodiment, method 1000 may include an additionaloperation 1610 as shown in FIG. 16.

Operation 1610 depicts receiving one or more scattering signals. Forexample, as shown in FIGS. 3-4 and 7B, 8B and 9B, the CCD detector 116may receive the scattered light 119B and convert that light intoelectrical signals to be processed by a computing device 126. As shownin FIG. 7B, the detector 116 may include a collection pupil 124 havingapertures 125. A detector 116 including collection pupil 124 of FIG. 7Bmay be employed in combination with a light source 110 including theillumination pupil 122 of FIG. 7A. Alternately, as shown in FIG. 8B, thedetector 116 may include a collection pupil 124 having light receivingpixels 125. A detector 116 including collection pupil 124 of FIG. 8B maybe employed in combination with a light source 110 including theillumination pupil 122 of FIG. 8A. Alternately, as shown in FIG. 9B, thedetector 116 may include a collection pupil 124 having light receivingpixels 125. A detector 116 including the collection pupil 124 of FIG. 9Bmay be employed in combination with a light source 110 including theillumination pupil 122 of FIG. 9A. A computing device 126 may receiveelectronic signals generated by the CCD detector 116 indicative of thelight scattering characteristics of one or more scatterometry cells 127of the overlay target 113. The computing device may compute one or moremetrology characteristics of a sample (e.g. overlay offset, criticaldistance and the like) from the electronic signals.

In an alternate embodiment, method 1000 may include additionaloperations 1710 and 1720 as shown in FIG. 17.

Operation 1710 depicts obtaining a third anti-symmetric differentialsignal (ΔS₃) associated with a third scatterometry cell. For example, asshown in FIGS. 1-4, a light source 110 may illuminate a thirdscatterometry cell 127C of an overlay target 113 with a first beam ofincident light 118 (e.g. a beam having a first polarization) to generatescattered light 119. A CCD detector 116 may receive the scattered light119 and convert that light into electrical signals to be processed by acomputing device 126. Further, the light source 110 may illuminate thethird scatterometry cell 127C of an overlay target 113 with a secondbeam of incident light 118 (e.g. a beam having a second polarization) togenerate scattered light 119. A CCD detector 116 may receive thescattered light 119 and convert that light into electrical signals to beprocessed by a computing device 126. The computing device 126 mayreceive the electronic signal responses associated with eachillumination of the scatterometry cell 127C and compute a differentialbetween the signals.

Operation 1720 depicts computing an overlay offset from the secondanti-symmetric differential signal (ΔS₂) associated with the secondscatterometry cell and a third anti-symmetric differential signal (ΔS₃)associated with a third scatterometry cell. For example, in the case ofthe second cell 127B and a third cell 127C of a overlay target 113having programmed offsets v₂=f₀ and v₃=−f₀ in a particular direction anddifferential signals ΔS₂ (as determined at Operation 1020) and ΔS₃ (asdetermined at Operation 1710) which are anti-symmetric, if both theoverlay and the programmed offsets are small, it may be assumed that thedifferential signals are linear functions of the total offsets:ΔS ₂ ∝f ₀+overlay;ΔS ₃ ∝−f ₀+overlay.Therefore, the overlay offset a cell pair 128B may be computed as:

${overlay} = {f_{0} \cdot {\frac{{\Delta\; S_{2}} + {\Delta\; S_{3}}}{{\Delta\; S_{2}} - {\Delta\; S_{3}}}.}}$

The foregoing described embodiments of the invention may be provided asillustrations and descriptions. They may be not intended to limit theinvention to precise form described. For example, many of the methodsdescribed herein in the context of scatterometry apply equally topolarized reflectometry. Other variations and embodiments may bepossible in light of above teachings, and it may be thus intended thatthe scope of invention not be limited by this Detailed Description, butrather by the appended claims.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowchartsand/or examples. Insofar as such block diagrams, flowcharts and/orexamples contain one or more functions and/or operations, it will beunderstood by those within the art that each function and/or operationwithin such block diagrams, flowcharts or examples can be implemented,individually and/or collectively, by a wide range of hardware, software,firmware or virtually any combination thereof. In one embodiment,several portions of the subject matter described herein may beimplemented via Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs), digital signal processors (DSPs), orother integrated formats. However, those skilled in the art willrecognize that some aspects of the embodiments disclosed herein, inwhole or in part, can be equivalently implemented in integratedcircuits, as one or more computer programs running on one or morecomputers (e.g., as one or more programs running on one or more computersystems), as one or more programs running on one or more processors(e.g., as one or more programs running on one or more microprocessors),as firmware, or as virtually any combination thereof, and that designingthe circuitry and/or writing the code for the software and or firmwaremay be well within the skill of one of skill in the art in light of thisdisclosure. In addition, those skilled in the art will appreciate thatthe mechanisms of the subject matter described herein may be capable ofbeing distributed as a program product in a variety of forms, and thatan illustrative embodiment of the subject matter described hereinapplies regardless of the particular type of signal bearing medium usedto actually carry out the distribution. Examples of a signal bearingmedium include, but may be not limited to, the following: a recordabletype medium such as a floppy disk, a hard disk drive, a Compact Disc(CD), a Digital Video Disk (DVD), a digital tape, a computer memory,etc.; and a transmission type medium such as a digital and/or an analogcommunication medium (e.g., a fiber optic cable, a waveguide, a wiredcommunications link, a wireless communication link (e.g., transmitter,receiver, transmission logic, reception logic, etc.), etc.).

In a general sense, those skilled in the art will recognize that thevarious aspects described herein which could be implemented,individually and/or collectively, by a wide range of hardware, software,firmware, or any combination thereof can be viewed as being composed ofvarious types of “electrical circuitry.” Consequently, as used herein“electrical circuitry” includes, but may be not limited to, electricalcircuitry having at least one discrete electrical circuit, electricalcircuitry having at least one integrated circuit, electrical circuitryhaving at least one application specific integrated circuit, electricalcircuitry forming a general purpose computing device configured by acomputer program (e.g., a general purpose computer configured by acomputer program which at least partially carries out processes and/ordevices described herein, or a microprocessor configured by a computerprogram which at least partially carries out processes and/or devicesdescribed herein), electrical circuitry forming a memory device (e.g.,forms of random access memory), and/or electrical circuitry forming acommunications device (e.g., a modem, communications switch, oroptical-electrical equipment). Those having skill in the art willrecognize that the subject matter described herein may be implemented inan analog or digital fashion or some combination thereof.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It may be to be understood that such depicted architecturesmay be merely exemplary, and that in fact many other architectures maybe implemented which achieve the same functionality. In a conceptualsense, any arrangement of components to achieve the same functionalitymay be effectively “associated” such that the desired functionality maybe achieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality may be achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected”, or“operably coupled”, to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable”, to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but maybe not limited to physically mateable and/or physically interactingcomponents, and/or wirelessly interactable, and/or wirelesslyinteracting components, and/or logically interacting, and/or logicallyinteractable components.

In some instances, one or more components may be referred to herein as“configured to,” “configurable to,” “operable/operative to,”“adapted/adaptable,” “able to,” “conformable/conformed to,” etc. Thoseskilled in the art will recognize that “configured to” can generallyencompass active-state components and/or inactive-state componentsand/or standby-state components, unless context requires otherwise.While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims may be toencompass within their scope all such changes and modifications as maybe within the true spirit and scope of the subject matter describedherein. It will be understood by those within the art that, in general,terms used herein, and especially in the appended claims (e.g., bodiesof the appended claims) may be generally intended as “open” terms (e.g.,the term “including” should be interpreted as “including but not limitedto,” the term “having” should be interpreted as “having at least,” theterm “includes” should be interpreted as “includes but may be notlimited to,” etc.). It will be further understood by those within theart that if a specific number of an introduced claim recitation may beintended, such intent will be explicitly recited in the claim and in theabsence of such recitation, no such intent may be present. For example,as an aid to understanding, the following appended claims may containusage of the introductory phrases “at least one” and “one or more” tointroduce claim recitations. However, the use of such phrases should notbe construed to imply that the introduction of a claim recitation by theindefinite articles “a” or “an” limits any particular claim containingsuch introduced claim recitation to claims containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation may be explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to “at least one of A, B, and C, etc.” may be used, in generalsuch a construction may be intended in the sense one having skill in theart may understand the convention (e.g., “a system having at least oneof A, B, and C” may include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to “at least one of A, B, or C, etc.” may be used,in general such a construction may be intended in the sense one havingskill in the art may understand the convention (e.g., “a system havingat least one of A, B, or C” may include but not be limited to systemsthat have A alone, B alone, C alone, A and B together, A and C together,B and C together, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that typically a disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be typicallyunderstood to include the possibilities of “A” or “B” or “A and B.”

With respect to the appended claims, those skilled in the art willappreciate that recited operations therein may generally be performed inany order. Also, although various operational flows may be presented ina sequence(s), it should be understood that the various operations maybe performed in other orders than those which may be illustrated, or maybe performed concurrently. Examples of such alternate orderings mayinclude overlapping, interleaved, interrupted, reordered, incremental,preparatory, supplemental, simultaneous, reverse, or other variantorderings, unless context dictates otherwise. With respect to context,even terms like “responsive to,” “related to,” or other past-tenseadjectives may be generally not intended to exclude such variants,unless context dictates otherwise.

Although specific dependencies have been identified in the claims, itmay be to be noted that all possible combinations of the features of theclaims may be envisaged in the present application, and therefore theclaims may be to be interpreted to include all possible multipledependencies. It may be believed that the present disclosure and many ofits attendant advantages will be understood by the foregoingdescription, and it will be apparent that various changes may be made inthe form, construction and arrangement of the components withoutdeparting from the disclosed subject matter or without sacrificing allof its material advantages. The form described may be merelyexplanatory, and it may be the intention of the following claims toencompass and include such changes.

What is claimed:
 1. A method for determining an overlay offsetcomprising: obtaining a first anti-symmetric differential signalassociated with a first scatterometry cell; obtaining a secondanti-symmetric differential signal associated with a secondscatterometry cell; obtaining a third anti-symmetric differential signalassociated with a third scatterometry cell; computing an overlay offset,in a first direction, from the first anti-symmetric differential signalassociated with the first scatterometry cell and the secondanti-symmetric differential signal associated with the secondscatterometry cell; and computing an overlay offset, in a seconddirection orthogonal to the first direction, from the secondanti-symmetric differential signal associated with the secondscatterometry cell and the third anti-symmetric differential signalassociated with the third scatterometry cell.
 2. The method of claim 1,wherein the first scatterometry cell comprises two-dimensional gratingpattern having: a first programmed offset in a first direction; and asecond non-zero programmed offset in a second direction orthogonal tothe first direction, and wherein the second scatterometry cell comprisesa two-dimensional grating pattern having: a first programmed offset inthe first direction unequal to the first programmed offset of the firstscatterometry cell; and a second non-zero programmed offset in thesecond direction equal to the second programmed offset of the firstscatterometry cell.
 3. The method of claim 1, further comprising:receiving one or more scattering signals.
 4. The method of claim 1,wherein the obtaining a first anti-symmetric differential signalassociated with a first scatterometry cell includes: illuminating thefirst scatterometry cell via an illumination beam having a firstpolarization; collecting scattering signals via an analyzer having asecond polarization orthogonal to the first polarization; illuminatingthe first scatterometry cell via an illumination beam having the secondpolarization; and collecting scattering signals via an analyzer havingthe first polarization; wherein the obtaining a second anti-symmetricdifferential signal associated with a second scatterometry cellincludes: illuminating the second scatterometry cell via an illuminationbeam having the first polarization; collecting scattering signals via ananalyzer having the second polarization; illuminating the secondscatterometry cell via an illumination beam having the secondpolarization; and collecting scattering signals via an analyzer havingthe first polarization.
 5. The method of claim 1, further comprising:providing one or more illumination beams from a source.
 6. The method ofclaim 4, wherein the first polarization is a first linear polarization;and wherein the second polarization orthogonal to the first polarizationis a second linear polarization.
 7. The method of claim 5, wherein theproviding one or more illumination beams from a source comprises:providing one or more illumination beams from a source having an atleast partially annular illumination pupil.
 8. The method of claim 5,wherein the providing one or more illumination beams from a sourcecomprises: providing one or more illumination beams from a source havingan at least partially apodized illumination pupil.
 9. The method ofclaim 5, wherein the providing one or more illumination beams from asource comprises: providing a monochromatic illumination beam from asource.
 10. The method of claim 5, wherein the providing one or moreillumination beams from a source comprises: providing an illuminationbeam from a source comprising a sequence of wavelengths.
 11. A systemcomprising: at least one processor device; and one or more instructionsthat, when implemented in the processor device, configure the at leastone processor for: obtaining a first anti-symmetric differential signalassociated with a first scatterometry cell; obtaining a secondanti-symmetric differential signal associated with a secondscatterometry cell; obtaining a third anti-symmetric differential signalassociated with a third scatterometry cell; computing an overlay offset,in a first direction, from the first anti-symmetric differential signalassociated with the first scatterometry cell and the secondanti-symmetric differential signal associated with the secondscatterometry cell; and computing an overlay offset, in a seconddirection orthogonal to the first direction, from the secondanti-symmetric differential signal associated with the secondscatterometry cell and the third anti-symmetric differential signalassociated with the third scatterometry cell.
 12. The system of claim11, wherein the obtaining a first anti-symmetric differential signalassociated with a first scatterometry cell includes: illuminating thefirst scatterometry cell via an illumination beam having a firstpolarization; collecting scattering signals via an analyzer having asecond polarization orthogonal to the first polarization; illuminatingthe first scatterometry cell via an illumination beam having the secondpolarization; and collecting scattering signals via an analyzer havingthe first polarization; wherein the obtaining a second anti-symmetricdifferential signal associated with a second scatterometry cellincludes: illuminating the second scatterometry cell via an illuminationbeam having the first polarization; collecting scattering signals via ananalyzer having the second polarization; illuminating the secondscatterometry cell via an illumination beam having the secondpolarization; and collecting scattering signals via an analyzer havingthe first polarization.
 13. The system of claim 12, wherein the firstpolarization is a first linear polarization; and wherein the secondpolarization orthogonal to the first polarization is a second linearpolarization.
 14. The system of claim 12, wherein the first polarizationis an azimuthal polarization; and wherein the second polarizationorthogonal to the first polarization is a radial polarization.
 15. Asystem comprising: circuitry for obtaining a first anti-symmetricdifferential signal associated with a first scatterometry cell;circuitry for obtaining a second anti-symmetric differential signalassociated with a second scatterometry cell; circuitry for obtaining athird anti-symmetric differential signal associated with a thirdscatterometry cell; circuitry for computing an overlay offset, in afirst direction, from the first anti-symmetric differential signalassociated with the first scatterometry cell and the secondanti-symmetric differential signal associated with the secondscatterometry cell; and circuitry for computing an overlay offset, in asecond direction orthogonal to the first direction, from the secondanti-symmetric differential signal associated with the secondscatterometry cell and the third anti-symmetric differential signalassociated with the third scatterometry cell.
 16. The system of claim15, wherein the obtaining a first anti-symmetric differential signalassociated with a first scatterometry cell includes: illuminating thefirst scatterometry cell via an illumination beam having a firstpolarization; collecting scattering signals via an analyzer having asecond polarization orthogonal to the first polarization; illuminatingthe first scatterometry cell via an illumination beam having the secondpolarization; and collecting scattering signals via an analyzer havingthe first polarization; wherein the obtaining a second anti-symmetricdifferential signal associated with a second scatterometry cellincludes: illuminating the second scatterometry cell via an illuminationbeam having the first polarization; collecting scattering signals via ananalyzer having the second polarization; illuminating the secondscatterometry cell via an illumination beam having the secondpolarization; and collecting scattering signals via an analyzer havingthe first polarization.
 17. A method comprising: obtaining a firstanti-symmetric differential signal associated with a first scatterometrycell, wherein the obtaining a first anti-symmetric differential signalassociated with a first scatterometry cell includes: illuminating thefirst scatterometry cell via an illumination beam having an azimuthalpolarization; collecting scattering signals via an analyzer having aradial polarization; illuminating the first scatterometry cell via anillumination beam having the radial polarization; and collectingscattering signals via an analyzer having the azimuthal polarization;and obtaining a second anti-symmetric differential signal associatedwith a second scatterometry cell, wherein the obtaining a secondanti-symmetric differential signal associated with a secondscatterometry cell includes: illuminating the second scatterometry cellvia an illumination beam having the azimuthal polarization; collectingscattering signals via an analyzer having the radial polarization;illuminating the second scatterometry cell via an illumination beamhaving the radial polarization; and collecting scattering signals via ananalyzer having the azimuthal polarization; and computing an overlayoffset, in a first direction, from the first anti-symmetric differentialsignal associated with the first scatterometry cell and the secondanti-symmetric differential signal associated with the secondscatterometry cell.